Pistons of the kind comprising a guide and sealing-ring located in an annular groove to extend around the piston circumference, and where the sealing-ring is biassed radially outwardly by a resilient bearing ring, comprise a flat guide and sealing ring, the cross-section of which is an elongated rectangle. The ring is located around an O-ring or a so-called "quad-ring" which biasses the guide or sealing ring radially outwards. As indicated by their name, these rings combine the function of a guide element--guiding the piston along the cylinder wall--with a sealing function, in that the two chambers which are separated from each other by the piston, are isolated one against the other. However, these known assemblies do not offer optimum sealing properties, quite apart from the fact that the guide and sealing rings, being rigid elements, may tilt or cant under certain conditions, and thus not only diminish the sealing effect but also adversely effect the guiding function of the ring.
With other known assemblies the piston is provided with two individual rings, one serving as a sealing ring and the other as a guide ring. With these assemblies however expenditure is excessive.
According to the present invention there is provided a piston for a pneumatic cylinder, said piston comprising a guide and sealing annular member extending around the circumference of the piston, and a resilient bearing member biasing the guide and sealing member in a direction for sealing engagement with the internal surface of the cylinder, the guide and sealing member comprising a plurality of axially consecutive, radially projecting sections for surface contact with the internal surface of the cylinder each pair of adjacent sections defining a cavity therebetween.
Preferably the sections of the guide and sealing member project radially by the same distance beyond a mounting assembly of the piston, the radially outer surfaces of the sections being contained in one common envelope plane. The external surfaces of the sections may be planar. The cavities between adjacent sections of the guide and sealing member define a hinge-like connection therebetween, the recesses which may for example open outwardly, having at their bases the contour of a circular arc. The advantages gained when using the new piston are its improved sealing power and its adaptability to discontinuities in the cylinder wall or varying cylinder tube diameters, and finally its improved guiding function which is consequential of its increased dimensions. The new assembly thus resembles a multiple piston ring type seal which includes between its individual seals, recessed spaces serving for the geometrical reduction of the material cross-section. The individual sealing sections thus have a guaranteed, radially elastic, function because of their hinge-like effect, and ensure optimum sealing conditions. The free spaces between the ring sections may for example be used as lubricant reservoirs. The guide and sealing member consists advantageously of a plastics material which offers the required sliding properties, for example a polyacetal. Preferably a plastics material is used which is commercially available under the name of "Hostaflon", a teflon and carbon alloy. By choosing the most suitable plastics material with optimum properties the multiple function sealing ring may offer additionally a satisfactory sliding power and thus may take up the function of a guide element.
The arrangement may for example be such that the resilient bearing ring comprises a plurality of axially consecutive bearing ring sections which are interconnected by thinner webs, the axially outermost sections being located opposite the axially outermost sections of the guide and sealing member.
Preferably the bearing ring is located within an opening defined in the mounting assembly and the bearing ring sections extend from one axial side of the opening to the other, and, while supported on the base of the opening serve as a bearing for the guide and sealing member. When a guide and sealing member is composed of five sections, the resilient bearing ring should preferably have three bearing ring sections. In this way it is ensured that the guide and sealing member is firmly and reliably pressed against the internal cylinder wall. The bearing ring may thereby consist of an elastically soft plastics material, for example of the known, commercially available, "Peruban".
It is especially advantageous when the axial length of the guide and sealing member and of the bearing ring are the same, and when the distance they cover in the axial direction exceeds that covered by known, comparable rings, being for example, 2 to 3 times as great. The advantage of the new arrangement is that the guide and sealing member is protected against canting by its increased axial length and that therefore a satisfactory and reliable piston control may be guaranteed. The good sealing properties of the ring are thus complemented by a very satisfactory piston guidance.